Nicotinic acetylcholine receptors (nAChRs) belong to the super family of ligand gated ionic channels, and gate the flow of cations including calcium. The nAChRs are endogenously activated by acetylcholine (ACh) and can be divided into nicotinic receptors of the neuromuscular junction and neuronal nicotinic receptors (NNRs). The NNRs are widely expressed throughout the central nervous system (CNS) and the peripheral nervous system (PNS). The NNRs have been suggested to play an important role in CNS function by modulating the release of many neurotransmitters, for example, ACh, norepinephrine, dopamine, serotonin, and GABA, among others, resulting in a wide range of physiological effects.
Seventeen subunits of nAChRs have been reported to date, which are identified as α2-α10, β1-β4, γ, δ and ε. From these subunits, nine subunits, α2 through α7 and β2 through β4, prominently exist in the mammalian brain. Many functionally distinct nAChR complexes exist, for example five α7 subunits can form a receptor as a homomeric functional pentamer or combinations of different subunits can form heteromeric receptors such as α4β2 and α3β4 receptors (Gotti, C. et al., Prog. Neurobiol., 2004, 74: 363-396; Gotti, C. et al., Biochemical Pharmacology, 2009, 78: 703-711)
The homomeric α7 receptor is one of the most abundant NNRs, along with α4β2 receptors, in the brain, wherein it is heavily expressed in the hippocampus, cortex, thalamic nuclei, ventral tegmental area and substantia nigra (Broad, L. M. et al., Drugs of the Future, 2007, 32(2): 161-170, Poorthuis R B, Biochem Pharmacol. 2009, 1; 78(7):668-76).
The role of α7 NNR in neuronal signalling has been actively investigated. The α7 NNRs have been demonstrated to regulate interneuron excitability and modulate the release of excitatory as well as inhibitory neurotransmitters. In addition, α7 NNRs have been reported to be involved in neuroprotective effects in experimental models of cellular damage (Shimohama, S., Biol Pharm Bull. 2009, 32(3):332-6).
Studies have shown that α7 subunits, when expressed recombinant in-vitro, activate and desensitize rapidly, and exhibit relatively higher calcium permeability compared to other NNR combinations (Papke, R. L. et al., J Pharmacol Exp Ther. 2009, 329(2):791-807).
The NNRs, in general, are involved in various cognitive functions, such as learning, memory and attention, and therefore in CNS disorders, e.g. Alzheimer's disease (AD), Parkinson's disease (PD), attention deficit hyperactivity disorder (ADHD), Tourette's syndrome, schizophrenia, bipolar disorder, pain and tobacco dependence (Keller, J. J. et al., Behav. Brain Res. 2005, 162: 143-52; Haydar, S. N. et al., Curr Top Med Chem. 2010; 10(2):144-52).
The α7 NNRs in particular, have also been linked to cognitive disorders including, for example, ADHD, autism spectrum disorders. AD, mild cognitive impairment (MCI), age associated memory impairment (AAMI) senile dementia, frontotemporal lobar degeneration, HIV associated dementia (HAD), HIV associated cognitive impairment (HIV-CI), Pick's disease, dementia associated with Lewy bodies, cognitive impairment associated with Multiple Sclerosis, Vascular Dementia, cognitive impairment in epilepsy, cognitive impairment associated with fragile X, cognitive impairment associated with Friedreich's Ataxia, and dementia associated with Down's syndrome, as well as cognitive impairment associated with schizophrenia. In addition, α7-NNRs have been shown to be involved in the neuroprotective effects of nicotine both in vitro (Jonnala, R. B. et al., J. Neurosci. Res., 2001, 66: 565-572) and in vivo (Shimohama, S., Brain Res., 1998, 779: 359-363) as well as in pain signalling. More particularly, neurodegeneration underlies several progressive CNS disorders, including, but not limited to, AD, PD, amyotrophic lateral sclerosis, Huntington's disease, dementia with Lewy bodies, as well as diminished CNS function resulting from traumatic brain injury. For example, the impaired function of α7 NNRs by beta-amyloid peptides linked to AD has been implicated as a key factor in development of the cognitive deficits associated with the disease (Liu, Q.-S., et al., PNAS, 2001, 98: 4734-4739). Thus, modulating the activity of α7 NNRs demonstrates promising potential to prevent or treat a variety of diseases indicated above, such as AD, other dementias, other neurodegenerative diseases, schizophrenia and neurodegeneration, with an underlying pathology that involves cognitive function including, for example, aspects of learning, memory, and attention (Thomsen, M. S. et al., Curr Pharm Des 2010 January; 16(3):323-43; Olincy, A. et al., Arch Gen Psychiatry. 2006, 63(6):630-8; Deutsch, S. I., Clin Neuropharmacol. 2010, 33(3):114-20; Feuerbach, D., Neuropharmacology. 2009, 56(1): 254-63)
The NNR ligands, including α7 ligands, have also been implicated in weight control, diabetis inflammation, obsessive-compulsive disorder (OCD), angiogenesis and as potential analgesics (Marrero, M. B, et al., J. Pharmacol. Exp. Ther. 2010, 332(1):173-80; Vincler, M., Exp. Opin. Invest. Drugs, 2005, 14 (10): 1191-1198: Rosas-Ballina, M., J. Intern Med. 2009 265(6):663-79; Arias, H. R., Int. J. Biochem. Cell Biol. 2009, 41(7):1441-51; Tizabi, Y., Biol Psychiatry. 2002, 51(2):164-71).
Nicotine is known to enhance attention and cognitive performance, reduced anxiety, enhanced sensory gating, and analgesia and neuroprotective effects when administered. Such effects are mediated by the non-selective effect of nicotine at multiple nicotinic receptor subtypes. However., nicotine also exerts adverse events, such as cardiovascular and gastrointestinal problems (Karaconji, I. B. et al., Arh Hig Rada Toksikol. 2005, 56(4):363-71). Consequently, there is a need to identify subtype-selective compounds that retain the beneficial effects of nicotine, or an NNR ligand, while eliminating or decreasing adverse effects.
Examples of reported NNR ligands are α7 NNR agonists, such as DMXB-A, SSR180711 and ABT-107, which have shown some beneficial effects on cognitive processing both in rodents and humans (see for example, Hajos, M., et al., J. Pharmacol Exp Ther. 2005, 312: 1213-22; Olincy, A. et al., Arch Gen Psychiatry. 2006 63(6); 630-8; Pichat, P., et al., Neuropsychopharmacology. 2007 32(1):17-34; Bitner, R. S., J Pharmacol Exp Ther. 2010 1; 334(3):875-86). In addition, modulation of α7 NNRs have been reported to improve negative symptoms in patients with schizophrenia (Freedman, R. et al., Am J Psychiatry. 2008 165(8): 1040-7).
Despite the beneficial effects of NNR ligands, it remains uncertain whether chronic treatment with agonists affecting NNRs may provide suboptimal benefit due to sustained activation and desensitization of the NNRs, in particular the α7 NNR subtype. In contrast to agonists, administering a positive allosteric modulator (PAM) can reinforce endogenous cholinergic transmission without directly stimulating the target receptor. Nicotinic PAMs can selectively modulate the activity of ACh at NNRs, preserving the activation and deactivation kinetics of the receptor. Accordingly, α7 NNR-selective PAMs have emerged (Faghih, R., Recent Pat CNS Drug Discov. 2007, 2(2):99-106).
Consequently, it would be beneficial to increase α7 NNR function by enhancing the effect of the endogenous neurotransmitter acetylcholine via PAMs. This could reinforce the endogenous cholinergic neurotransmission without directly activating α7 NNRs, like agonists. Indeed, PAMs for enhancing channel activity have been proven clinically successful for GABAa receptors where benzodiazepines and barbiturates, behave as PAMs acting at distinct sites (Hevers, W. et al., Mol. Neurobiol, 1998, 18: 35-86).
To date, only a few NNR PAMs are known, such as 5-hydroxyindole (5-HI), ivermectin, galantamine, and SLURP-1, a peptide derived from acetylcholinesterase (AChE). Genistein, a kinase inhibitor was also reported to increase α7 responses. PNU-120596, a urea derivative, was reported to increase the potency ACh as well as improve auditory gating deficits induced by amphetamine in rats. Also, NS1738, JNJ-1930942 and compound 6 have been reported to potentiate the response of ACh and exert beneficial effect in experimental models of sensory and cognitive processing in rodents. Other NNR PAMs include derivatives of quinuclidine, indole, benzopyrazole, thiazole, and benzoisothiazoles (Hurst, R. S. et al., J. Neurosci. 2005, 25: 4396-4405; Faghih, R., Recent Pat CNS Drug Discov. 2007, 2(2):99-106; Timmermann, D. B., J. Pharmacol. Exp. Ther. 2007, 323(1):294-307; Ng, H. J. et al., Proc. Nat. Acad. Sci. USA. 2007, 8; 104(19):8059-64; Dinklo, T., J. Pharmacol. Exp. Ther. 2011, 336(2):560-74.).
WO 2009/043784 recites compounds of the overall structure
which compounds are said to be PAMs of the α7 NNR.
The α7 NNR PAMs presently known generally demonstrate weak activity, have a range of non-specific effects, or can only achieve limited access to the central nervous system where α7 NNRs are abundantly expressed. Accordingly, it would be beneficial to identify and provide new PAM compounds of α7 NNRs and compositions for treating diseases and disorders wherein α7 NNRs are involved. It would further be particularly beneficial if such compounds can provide improved efficacy of treatment while reducing adverse effects associated with compounds targeting neuronal nicotinic receptors by selectively modulating α7 NNRs.
WO 2010/137351 recites compounds of the overall structure
as calcium or sodium channel blockers i.e. compounds related to a pharmacological mechanism distinguished from the compounds of the present invention.